The present invention relates to a display medium which may be provided on a card, a label, a seal, etc. More particularly, the present invention relates to a display medium employing a diffraction grating to define a display pattern, e.g., a character, an image, etc., by the contour thereof, thereby enabling such a display pattern to move as the visual point moves. The present invention also relates to a method of producing a diffraction grating assembly for the display medium.
A technique of forming a display pattern, e.g., a character, an image, etc., by the contour of a diffraction grating has heretofore been known, for example, as a transparent diffraction grating label proposed by the present applicant in U.S. Pat. No. 5,184,848. However, such conventional technique cannot enable a display pattern, e.g., a character, an image, etc., to move as the observer moves the visual point because the display pattern is displayed by the contour of a simple diffraction grating.
On the other hand, Japanese Patent Application Laid-Open (KOKAI) No. 2-72320 proposes a display medium comprising two-dimensionally arranged minute dots, which are respectively made of diffraction gratings, wherein the diffraction gratings of the dots are varied in direction and pitch so that the display pattern changes according to the viewing direction. In this display medium, however, the display pattern is not defined by the contour of a diffraction grating, and the change occurring in the display pattern as the visual point moves is not smooth. Moreover, it is not easy to produce the display medium because a single display pattern is formed by regularly arranging an extremely large number of dots as in the case of halftone dots.